The contamination of water by heavy metals has been an increasingly important issue in separation science and environmental remediation. The prominent heavy metal pollutants, such as Hg2+, Pb2+ and Cd2+, in some natural water sources and industrial waste water constitute a threat for humans and other species.1-3 Ion exchange and chemical precipitation are traditional methods for removing these heavy metals.4, 5 However, effective removal of heavy metal ions at low metal concentrations remains a great challenge.6 Precipitation methods with sulfide ions cannot reduce the concentrations of heavy metals below acceptable drinking levels.7 Thus, new and highly efficient adsorbents and methodologies need to be developed.
Selective metal adsorption on suitable substrate materials is considered one of the most economical methods of removal or recovery.8 Natural adsorbents such as clays9, 10 and zeolites11 have been commonly employed because of their high surface area, hydrophilic character and low cost. These materials, however, suffer from low selectivity and weak affinity for heavy metal ions. Alternatively, sulfide-based materials12-16 seem to be effective for heavy metal remediation, as the high affinity of soft Lewis basic frameworks for the soft Lewis acids (e.g. Hg2+) is innate to these materials. Mineral sulfides, such as FeS2, have the disadvantage of adsorbing metal ions only on their surface, due to their dense structures.17-19 
Several functionalized layered materials have shown efficiency for heavy metal remediation.20-23 Synthetic layered sulfides, such as K2xMnxSn3-xS6 (KMS)24-29 or H2xMnxSn3-xS6,30 with the ability for removing heavy metal ions have been reported. These materials operate under the soft-hard Lewis acid-base paradigm for the metal selectivity.
Elemental mercury (Hg0) is a major toxic pollutant in the flue gas of coal combustion. The removal of mercury from this gaseous medium presents a great challenge due to the limitations in the conventional air-pollution control techniques such as fabric filtering (FF) and electrostatic precipitation (ESP).1a,2a These technologies rely on supporting materials such as activated carbon, activated alumina, and zeolites whose surface properties make them viable for mercury adsorption.3a,4a However, the poor chemical interaction between these materials and mercury reduces their adsorption capability, and the low loadings result in low Hg absorption capacity, which limits their application. Because of the high affinity of sulfur toward Hg0, sulfur-impregnated activated carbon and zeolites have been studied for mercury adsorption,5a and improvements in mercury capture efficiency have been achieved by functionalization with sulfide-containing groups,6a however, the contamination of the combustion products (fly ash) by the activated carbon is undesirable.
Non-carbon materials functionalized with sulfur, for example Co-doped iron nanoparticles7a and silica,8a porous silica,9a silica-titania nanocomposites,10a especially sodium polysulfide-montmorillonites,11a have recently demonstrated high mercury capture capacity.
The layered double hydroxides (LDHs), a known type of anionic clay, have positively-charged host layers and counter-anions in the interlayers. Thanks to their excellent exchangeability, LDHs can work as precursors to introduce other species to fabricate hybrid materials, which reveal important applications in adsorption,40,41 catalysis,42-45 separation science,46-49 storage and triggered release of functional guests,50-53 optical materials,54 etc. LDH materials have been studied for the removal of heavy metal ions, but in general they exhibit low selectivity.55-58 LDH intercalated with mercaptocarboxylic acid (with an S—H group) was reported to remove Hg2+,59 but with moderate efficiency, possibly due to the steric hindrance of carboxylic groups.
Other studies have focused on the adsorption of anion pollutants. LDHs functionalized with anionic compounds can form new hybrid structures to produce particular properties. For example, intercalation of macrocyclic cyclodextrins into LDHs24a-27a results in hydrophobic nanopockets with well-defined size and shape within the hydrophilic interlamellar space, allowing selective adsorption of neutral species such as I2,28a naphthalene,29a anthracene,30a ferrocene,31a dodecylbenzene,27a and phenol compounds.32a Some sulfur-impregnated LDH composites were reported to remove mercury ions.33a 
The radioactive metal-containing waste generated from the increasing use of nuclear power presents a threat to the environment, natural water resources, and human health. Uranium salts that come from nuclear fuel fabrication, ore mining, manufacturing and processing are major reasons that nuclear energy may be very harmful to the environment.1b 
Nuclear power will continue to be an important source of the world's electrical power in the coming decades but this is dependent upon finding, harvesting, and managing the required amount of uranium. One of the most abundant sources of uranium is seawater at an estimated 4×1012 kg (at ˜3-4 ppb).2b The primary issue becomes harvesting this uranium in a cost effective manner with high yield. For the recovery of this uranium from the sea, various adsorbents including synthetic polymers, inorganic materials, and biopolymers have been tested. All these strategies approach the problem under the hypothesis that the uranyl ion is a hard Lewis acid species.3b Uranium recovery is further hindered by the complex solution chemistry in seawater; uranium is present in multiple forms including [UO2(CO3)3]4−, [UO2(OH)3]−, [UO2(CO3)2]2−, [UO2]2+, UO2(OH)+, and UO2(OH)2, although the dominant form (˜85%) is the [UO2(CO3)3]4− complex. Furthermore, seawater has varying solution characteristics in terms of pH (7.5-8.5), temperature (2-40° C.), high salt concentration (0.6-0.7 M).
In recent years, a Japanese technology was developed to capture uranium from seawater using long mats of braided plastic fibers embedded with the uranium-adsorbent, amidoxime. These mats were placed underwater within the ocean and left for a period of time, after which they were removed and washed with an acidic solution that captured the uranium for future refinement.1b Currently, work is being done at Oak Ridge National Laboratory and Pacific Northwest National Laboratory (PNNL) to improve the adsorption capacity using fibrous sorbents with higher surface areas than those used by the Japanese. These studies show a >2× improvement in uranium adsorption capacity. Sorbents that show an even greater improvement in uranium adsorption capacity or uptake efficiency will be important to further reduce the cost of mining uranium from seawater.
For decreasing uranium concentrations, many technologies including ion exchange/absorption,4b-6b adsorption,7b-9b and chemical/biochemical reductive precipitation10b-13b have been developed. Inorganic ion-exchangers such as clays and zeolites are generally of higher chemical and thermal stability, as well as more affordable, compared to the organic resins, so many inorganic exchangers used as absorbents for heavy metals (e.g., UO22+) have been investigated.14b,15b But they have some drawbacks resulting from the slow exchange kinetics between the inorganic exchangers and large hydrated cations such as [Sr(H2O)x]2+ and [UO2(H2O)x]2+. This limits their applicability for effective treatment of contaminated solutions.16b 
Uranium exists mainly as the uranyl cation UO22+ in aqueous solutions, which is a hard cation in the Lewis acid sense based on its hexavalent oxidation state and the presence of O atoms.1b However, compounds with uranyl cations can form strong covalent bonds with soft S2− groups.17b The mineral sulfides (such as FeS2) with soft ligands can act as uranyl scavengers, through reduction of the uranium(VI) and precipitation of the insoluble U3O8.18b,19b It has also been found that prepared layered sulfides KMS-1(K2xMnxSn3-xS6, x=0.5-0.95) showed good exchange/adsorption properties for UO22+.20b 